The obtained engine experimentation results with KME and KME-DTBP blends in the form of engine performance parameters, viz. BTE, BSEC and EGT along with exhaust emissions, viz. CO, HC, NOx and smoke opacity were analyzed critically and compared with those for diesel fuel. Again, the comparative analyses of the in-cylinder peak pressure with the selected blends along with diesel has been presented in this section for better understanding of the combustion pattern of these fuels. These comparative analyses carried out at selected engine loading conditions are presented in the following subsections along with valid discussions in order to ascertain the improvement/deterioration in the said performance and exhaust emissions characteristics.
4.1 In-cylinder Peak Pressure
The variations of in-cylinder peak pressure for KME, KMED1, KMED2, KMED3, KMED5, and diesel at 85% load and at rated load with respect to crank movement are presented in Figs. 2 (a) and (b), respectively. It is clearly observed that the highest peak pressure at both 85% and 100% loads is achieved with diesel that shows its better combustion pattern compared to the other fuels due to its higher calorific value and lower viscosity. Similarly, the lowest peak pressure is achieved with KME at both the mentioned loads, which signifies its poor combustion pattern compared to the other blends and diesel owing to its lower cetane index, calorific value and higher viscosity. It is also observed that the peak pressure at both considered loads increases with increment in DTBP percentage in the blends. Among all considered biodiesel blends, KMED5 exhibits highest peak pressure at both conditions. This may be credited to the higher calorific value, higher cetane index and lower viscosity of KMED5 compared to the other blends. The trend of combustion results signifies that use of additive with biodiesel improve the fuel properties, which in turn improve the combustion characteristics of the resulting blend. These results are in agreement with the published literature (Panda et al. 2017; Panda et al. 2018). Further, the combustion analysis reveals that KMED5 is the most suitable fuel among all the considered biodiesel blends owing to its better fuel properties.
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(a) At 85% load
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(b) At 100% load
Figure 2. Variation of in-cylinder peak pressure
4.2 Brake Thermal Efficiency
Figure 3 demonstrates the variation of BTE with load. It is detected that BTE increases initially up to 85% load and then somewhat decreases until full load irrespective of the type of fuel used. This shows better combustion at higher loads, which is the common trend of a CI engine. At higher loads, the amount of fuel supplied is more that leads to higher brake power and BTE (Devarajan et al. 2019b). The BTE was found to be highest for diesel at all loads. Among the biodiesel fuels, KMED5 showed highest BTE at all loads. This may be credited to the higher cetane index and calorific value along with lower viscosity of KMED5 that resulted in better atomization, mixing and superior combustion compared to other biodiesel fuels. It is further observed that addition of DTBP with KME resulted in better combustion and higher BTE compared to neat biodiesel. These results are in covenant with published literature (Devarajan et al. 2018). At 85% load, KMED5 showed the highest BTE among the biodiesel fuels that was a marginal 2.35% lower than that for diesel. Again, at full load, the BTE with KMED5 was the highest amid all the biodiesels and was lesser by only 2.8% compared to that with diesel.
Figure 3. Variation of BTE with load
4.3 Brake Specific Energy Consumption
The variation of BSEC with load is presented in Fig. 4. It is noticed that BSEC initially decreases up to 85% load and then marginally increases until full load with all the fuels. Higher BSEC at low loads is owing to incomplete combustion because of lower in-cylinder pressure, low turbulence and poor mixing. On the other hand, higher BSEC at high loads is because of rich mixture formation due to increased fuel supply quantity. BSEC with diesel was found to be lowest at all loads. This may be because of higher calorific value of diesel that leads to better combustion efficiency and lower fuel energy consumption (Devarajan et al. 2018). The BSEC trend also shows that with increase in DTBP percentage in KME, BSEC gradually reduces. This signifies better combustion efficiency with addition of DTBP in KME. The same may be attributed to the increase in cetane index and calorific value as well as reduction in viscosity with addition of DTBP in the blend that leads to improved atomization and superior combustion. These results are well supported by the findings of the published literature (Yilmaz and Atmanli 2017). Among all selected biodiesel fuels, KMED5 showed lowest BSEC at all loads. At 85% load, BSEC with KMED5 was found to be only 1.09% higher than that with diesel. Again, the same for KMED5 was found to be 2.34% higher matched to that with diesel.
Figure 4. Variation of BSEC with load
4.4 Exhaust Gas Temperature
The variation in EGT with load is depicted in Fig. 5. EGT is found to be increasing with rise in load irrespective of the fuel used. With increase in load, the in-cylinder pressure and temperature tends to increase those result in higher EGT. Diesel produces lowest EGT at all loads. Biodiesel, being an oxygenated fuel, produces higher combustion temperature, which in turn results in higher EGT (Yilmaz and Atmanli 2017; Xue et al. 2011). It is further detected that upsurge in DTBP percentage in KME leads to higher EGT. The probable reason for this is the increase in cetane index and calorific value of KME with increase in DTBP content in the blend. The same results in early start of combustion and a prolonged secondary-phase combustion leading to higher EGT. These findings and their explanations are in agreement with the published literature (Xue et al. 2011). The highest EGT were observed at full load with all the fuels. At this load, the lowest EGT of 447.6°C was obtained with diesel and the highest of 508°C was obtained for KMED5. Again, EGT for diesel was found to be 7.9%, 8.8%, 9.7%, 10.7%, and 13.5% lower compared to that with KME, KMED1, KMED2, KMED3, and KMED5 respectively, at full load.
Figure 5. Variation of EGT with load
4.5 Carbon Monoxide
Figure 6 depicts the variation in CO emissions with load. It is detected that the CO emissions for all the selected fuels are slightly on the higher side at lower engine loads, are lowest at 85% load and are significantly higher after 85% load. At lower engine loads, incomplete combustion takes place due to lower in-cylinder temperature and pressure as well as less turbulence. Because of this, the CO emissions are slightly on the higher side (Gharehghani and Pourrahmani 2019). Further, lowest CO emissions at 85% load are due to better combustion. On the other hand, sharp increase in CO emissions observed after 85% load are mainly because of imperfect combustion owing to formation of rich mixture as a result of higher injection pressures along with the higher in-cylinder temperature at which the CO2 molecules dissociate to form CO. These results settle with the outcomes in the published literature (Gharehghani and Pourrahmani 2019; Roy et al. 2013). It is further observed that blending of DTBP additive with KME results in reduction of CO emissions. The CO emissions tend to decrease with rise in percentage of the additive in KME. This may be credited to the enhancement of calorific value along with cetane index as well as decrease in viscosity of the fuel due to addition of the additive, which is reflected in Table 1. Roy et al. (2016) have verified that addition of cetane improving additive to biodiesel lowers CO emissions. The highest CO emissions were obtained for diesel, which is due to the availability of less oxygen, whereas the lowest was obtained with KMED5 blend among all selected fuels. At the best performing load (85%), the CO emissions with KMED5 was found to be 10.8%, 16.2%, 18.9%, 24.3% and 29.7% lower than those with KMED3, KMED2, KMED1, KME and diesel, respectively. Again, KMED5 was found to produce 13.22%, 18.18%, 33.88%, 38.02%, and 43.8% lower CO emissions compared to KMED3, KMED2, KMED1, KME and diesel, respectively. The lowest CO emissions of 0.037% and 0.121% were recorded with KMED5 at 85% and 100% load, respectively.
Figure 6. Variation of CO emissions with load
4.6 Unburned Hydrocarbons
The deviation in HC emissions with engine load is presented in Fig. 7. The obtained results show that the HC emissions initially decrease with rise in load up to 85% load and then increase at a stiff rate until full load irrespective of the type of fuel used. Higher HC emissions at lower range of loads is due to incomplete combustion that is caused by lower cylinder pressure and temperature, weak turbulence, improper mixing and less availability of oxygen (Roy et al. 2013; Dhar et al. 2012). The lowest HC emissions were observed at 85% load for all the fuels that shows better combustion at this load. The rapid growth in HC emissions for all the fuels after 85% load is attributed to formation of rich fuel-air mixture due to greater injection pressures leading to incomplete combustion (Radhakrishnan et al. 2017). The HC emissions with KME and its blends with DTBP was observed to be lower at all loads matched to diesel. This may be credited to the higher oxygen content in case of biodiesel and the higher cetane index of KME and its blends with DTBP that leads to improved combustion compared to diesel (Radhakrishnan et al. 2017; Doğan 2011). Again, the HC emissions in case of KME was observed to reduce with increase in percentage of DTBP. This is due to the mutual effect of decline in viscosity and enhancement in cetane index and calorific value that leads to improved atomization and superior combustion (Atmanli et al. 2014). In addition, increase in percentage of DTBP in KME lowers the ignition delay and enhances mixing of fuel and air. Consequently, the combustion rate is improved, which lowers the HC emissions (Kumar et al. 2016). The above-depicted results are in harmony with the work of Devarajan et al. (2017a). The lowest HC emissions were obtained with KMED5, whereas the same was found to be highest with diesel at all loads. At 85% load, the HC emissions with KMED5 was observed to be 24.3%, 36.2%, 42.8%, 45.3%, and 70.5% lower compared to KMED3, KMED2, KMED1, KME, and diesel, respectively. Similarly, 8.65%, 16.26%, 22.3%, 27.1%, and 58.02% lower HC emissions were recorded with KMED5 compared to KMED3, KMED2, KMED1, KME, and diesel respectively, at 100% load. The lowest HC emissions of 13.23 ppm and 36.78 ppm were recorded with KMED5 at 85% and 100% load, respectively.
Figure 7. Variation of HC emissions with load
4.7 Nitrogen Oxides
Nitrogen oxide is a common term for NO and NO2 expressed as the formula NOx. Mainly two types of NOx formation is believed to occur in the CI engine combustion chamber. They are thermal NOx (Zeldovich mechanism) and prompt NOx (Fenimore mechanism) (Musthafa 2017; Rahman et al. 2013). Major NOx formation is due to thermal NOx that primarily depends on the availability of oxygen, mixture temperature and a longer residence time of high temperature gases (Gharehghani and Pourrahmani 2019; Gharehghani et al. 2017; Özener 2014). The variation of NOx emissions for all the selected fuels are depicted in Fig. 8. It is noticed that the NOx emissions upsurge with rise in engine load for all the fuels. At higher loads, the in-cylinder temperature is higher due to combustion of larger quantity of fuel. This in turn, increases the residence time of higher temperature leading to higher NOx emissions (Devarajan et al. 2019b; Joy et al. 2019). Highest NOx emissions were observed at 100% load for all selected fuels. Again, biodiesels produce more NOx emissions compared to diesel (Musthafa 2017; Devarajan et al. 2019a). This is attributed to the higher oxygen content in biodiesels that leads to increased rate of combustion and higher temperature of combustion gases. Thus, the present trend of NOx emissions are found to be lowest for diesel among all the fuels at all engine loads. It is further witnessed that the NOx emissions slightly increase with increment in DTBP percentage in KME. Addition of DTBP to KME enhances the cetane index and calorific value, whereas it also reduces the viscosity of the blend. This result in a shorter delay period, early start of combustion, increased premixed combustion phase as well as improved atomization and mixing. The collective effect of all these factors leads to longer residence time of high temperature in the combustion chamber, giving rise to higher NOx emissions (Enweremadu and Rutto 2010; Pattanaik and Misra 2018; Pattanaik et al. 2017). Thus, KMED5 having highest DTBP percentage, exhibited highest NOx emissions among all the fuels at all loads, However, this increment in NOx emissions for KMED5 is found to be nominal. The highest NOx emissions of 1277.6 ppm and 1366.5 ppm were recorded with KMED5 at 85% and 100% load, respectively. On the other hand, diesel exhibited lowest NOx emissions of 1165.66 ppm and 1234.6 ppm at 85% and 100% load, respectively. At the best performing load of 85%, the NOx emissions with KMED5 was observed to be higher by 1.48%, 1.72%, 3.37%, 2.93%, and 8.76% compared to those with KMED3, KMED2, KMED1, KME, and diesel, respectively. Likewise, the same at 100% load was found to be 0.57%, 2.48%, 3.1%, 3.23%, and 9.65% higher compared to those with KMED3, KMED2, KMED1, KME, and diesel, respectively.
Figure 8. Variation of NOx emissions with load
4.8 Smoke Opacity
Diesel smoke is the combination of aerial particulate matter and gases produced during combustion. The smoke is measured in the smoke meter by means of light absorbed through an exhaust gas column of defined specific length. The same is expressed as smoke opacity. The variations of smoke opacity with engine load for diesel, KME, KMED1, KMED2, KMED3, and KMED5 are presented in Fig. 9. It is noticed that the smoke opacity linearly increases with rise in engine load irrespective of the type of fuel used. Further, the same is observed to be increasing significantly at higher loads. At higher loads, the amount of fuel supplied is greater in order to maintain a constant power output. This in turn, results in rich fuel-air mixtures that leads to incomplete combustion (Devarajan et al. 2017b; Mahalingam 2018). Hence, smoke opacity are significantly higher at high engine loads. Diesel exhibited higher smoke opacity at all loads compared to the biodiesel fuels. The same can be attributed to the existence of oxygen in biodiesels that ensures better combustion and less smoke formation compared to diesel. Again, addition of DTBP to KME showed reduction in smoke opacity. It is witnessed that smoke opacity tends to reduce with higher percentage of DTBP in KME. This may be attributed to the reduction in viscosity along with enhancement in cetane index due to increase in additive percentage in biodiesel. Reduction in viscosity improves the atomization process and produces better mixing of fuel with air. In addition, improved cetane index lowers the delay period that leads to early commencement of combustion. The combined effect of the same results in improved combustion in the primary combustion phase that produces lower smoke formation (Pandian 2017). KMED5 showed lowest smoke opacity among all the tested fuels at all loads. The lowest smoke opacity observed at 85% and 100% load are 21.45% and 40.12%, respectively. At 85% load, the smoke opacity of KMED5 was found to be 14.5%, 24.85%, 37.3%, 41.95%, and 65.82% lower compared to KMED3, KMED2, KMED1, KME, and diesel, respectively. Similarly, the same for KMED5 at 100% load was found to be lower by 6.58%, 10.24%, 14.33%, 16.0%, and 37.68% compared to KMED3, KMED2, KMED1, KME, and diesel, respectively.
Figure 9. Variation of smoke opacity with load